Speed sensor for a rotating member or machine

ABSTRACT

A speed sensor for use in measuring the speed of rotation of a rotationally salient rotating member. The speed sensor comprises an electrode and a sensor circuit. The sensor circuit comprises a constant voltage source for supplying a voltage to the electrode to generate an electric field in a dielectric medium. A current detector detects current flow between the constant voltage source and the electrode due to perturbation of the electric field by passage of at least one salient feature of the rotating member through the electric field as the rotating member rotates. The current detector outputs a first signal modulated at a frequency corresponding to the frequency of perturbation of the electric field. The first signal is amplified to produce an amplified signal modulated at a frequency corresponding to the frequency of perturbation of the electric field.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.12/618,963 filed on Nov. 16, 2009, which is a continuation ofPCT/GB2008/001674 filed on May 15, 2008, which claims priority to UnitedKingdom Application No. 0709397.4 filed May 15, 2007, all of which areincorporated herein by reference.

This application claims priority from United Kingdom application number0709397 the entire contents of which are hereby incorporated byreference.

The present invention relates to a sensor. Particularly, but notexclusively, the present invention relates to a speed sensor forturbomachinery, such as, for example, a turbocharger.

Turbochargers are well known devices for supplying air to the intake ofan internal combustion engine at pressures above atmospheric (boost)pressure. A conventional turbocharger essentially comprises an exhaustgas driven turbine wheel mounted on a rotatable shaft within a turbinehousing. Rotation of the turbine wheel rotates a compressor wheelmounted on the other end of the shaft within a compressor housing. Thecompressor wheel delivers compressed air to the intake manifold of theengine, thereby increasing engine power.

It is known to provide a turbomachine with a sensor to measure operatingcharacteristics of the turbomachine. Any such operational informationcan be used as one parameter of a turbomachine control system. Forexample, the distance between members of the turbomachine.

One type of sensor that has been used as a turbomachine sensor is acapacitive sensor comprising a resonant circuit including a capacitorformed between a sensor electrode mounted in a bore provided in a wallof the turbine housing and the electrically conductive turbine wheel. Asthe turbine wheel rotates, the capacitance fluctuates dependant on thedistance between the turbine wheel and the sensor electrode as eachblade sweeps past the electrode. The separation between the sensorelectrode and the turbine wheel is thus determined by detecting andamplifying frequency modulation in the resonant circuit due to thevarying capacitance.

Within known capacitive sensors the separation between the electrode andother turbomachine component, which forms the capacitor, must be kept toa minimum, as capacitance changes sharply with increasing separation, asa function of the inverse of the separation distance. However a minimumseparation is normally required for reasons of mechanical construction,tolerancing and allowance for thermal expansion. As such, signal levelsare often very low. In engine environments there is generally aconsiderable level of electrical noise, compared with a testinglaboratory which would generally have far lower noise levels. This lowsignal level and the electrical noise in the working environment resultsin a low signal to noise ratio and therefore it is often difficult toamplify the desired signal to the required levels whilst also rejectingthe noise from the signal. Capacitive sensors are also problematic inthat their operation can be adversely affected by the presence ofcontaminants in the gas passing through the turbocharger, as well as thepresence of contaminant deposits on the sensor electrode itself.

The measurement of other turbomachine operating characteristics mayprove of use. For instance, by providing information concerning thespeed of a turbocharger to an engine control unit (ECU) it may bepossible to prevent or counteract any turbocharger over speeding.

It is an object of the present invention to obviate or mitigate theabove disadvantages.

According to a first aspect of the present invention there is provided aspeed sensor for use in measuring the speed of rotation of arotationally salient rotating member, the speed sensor comprising anelectrode and a sensor circuit;

-   -   the sensor circuit comprising:    -   constant voltage source for supplying a voltage to the electrode        to generate an electric field in a dielectric medium;    -   a current detector for detecting current flow between the        constant voltage source and the electrode due to perturbation of        the electric field by passage of at least one salient feature of        the rotating member through the electric field as the rotating        member rotates, the current detector outputting a first signal        modulated at a frequency corresponding to the frequency of        perturbation of the electric field; and    -   an amplifier circuit comprising a signal amplifier for        amplifying the first signal and outputting a second signal        modulated at a frequency corresponding to the frequency of        perturbation of the electric field;        wherein the electrode voltage, the amplifier gain and the        electrode position relative to the rotating member are selected        such that modulation of the second signal is predominantly        caused by perturbation of the electric field within the        dielectric medium by the creation and/or movement of ions within        the field.

It will be appreciated that the constant voltage source is only requiredto operate at a constant voltage for a period of time sufficient for thesensor to detect the rotational speed of the rotating member. Theconstant voltage source may for instance be capable of outputting arange of voltages, which may change over a period of time.

In addition it will be appreciated that whilst the voltage source triesto maintain a constant voltage, the voltage at the electrode may in factfluctuate due to a lag between perturbation of the electric field andthe response of the voltage source. The term “constant” should beinterpreted accordingly as any such fluctuation may be disregarded.

It will be understood that a salient feature of the rotating member isany feature that causes said perturbation in the electric field as itpasses therethrough. A salient feature may for instance be a feature ofthe three dimensional configuration of the rotating member or alocalised change in a property of the material of the rotating member(including a localised change of material) or a combination of these.

The dielectric medium may for instance comprise a liquid or gas (e.g.air) between the electrode and the rotating member, or a non-conductivesolid (e.g. an electrically insulating plastics material), or acombination of two or more of these.

The sensor circuit may comprise a frequency divider for receiving thesecond signal and outputting a third signal.

The sensor circuit may comprise a frequency counter for receiving thesecond or third signals and supplying a third signal indicative of thefrequency of said perturbation. The sensor circuit may include an outputterminal or cable for connection to a controller or other means fordetermining the rotational speed of said rotating member from saidsecond, third or fourth output signals.

The sensor circuit may comprise means for determining the rotationalspeed of said rotating member from said first, second, third or fourthoutput signals.

The voltage source may supply a voltage greater than about +/−30, forinstance in the range of +/−30V to +/−500V or higher. In someembodiments of the invention voltages as high as +/−1 kV or more may beappropriate. In other embodiments a voltage in the range of +/−30V to+/−150V may be appropriate. One preferred voltage range is +/−50V to+/−150V, e.g. of the order of 120V.

For instance, a raised sensor voltage relative to a relevant groundpermits the described perturbation be detected, and offers improvedsignal to noise ratios. It has been discovered that voltages well inexcess of the typical voltages used in typical electronics circuitry(i.e. 5V or 12V) causes effects in the movement of charge which areconsistent with each revolution or the rotating member, whereas lowervoltages allow the electrode potential to fluctuate in response tocontaminants in a proximate fluid flow, or any variation in potential ofany relevant proximal ground due to poor electrical connections (such asdue to a shaft or wheel being connected to earth through an oil filmbearing).

At least a portion of the speed sensor may be electrically shielded byat least one conductive shield. For instance a connection between theelectrode and the sensor circuit (which may be remote from theelectrode) or between elements of the sensor circuit (which may beremote from one another) may be shielded.

The shield may be held at a controlled electric potential.

The amplifier circuit may include a shield amplifier the output of whichdrives the or each conductive shield.

The speed sensor may comprising at least two conductive shieldsshielding respective portions of the circuit.

The or each shield amplifier may hold each conductive shield at the sameelectric potential. The or each shield amplifier preferably has unitygain.

The or each shield amplifier may be coupled to the signal amplifier.

A feed back loop may be connected across the amplifier and including afirst capacitor. In one embodiment a shield amplifier is connected inseries with the signal amplifier, and a feed back loop is connectedacross the signal amplifier and said shield amplifier, the feedback loopincluding a first capacitor. The first capacitor may be part ofcapacitive divider which is adapted to reduce the effective capacitanceof the capacitor. The capacitive divider may comprise said firstcapacitor and a potential divider.

The gain of the amplifier circuit may be controlled by a capacitor witha value less than 100 pF, more preferably less than 10 pF, morepreferably less than 1 pF, more preferably less than 0.1 pF, or morepreferably still less than 0.01 pF. This capacitor may be in acapacitance divider as for instance mentioned above to provide a furtherreduced effective capacitance which is preferably less than 10 pF, morepreferably less than 1 pF, more preferably less than 0.1 pF, morepreferably less than 0.01 pF, and most preferably less than 0.001 pF.

In preferred embodiments the first output signal is a modulated voltage.

The current detector may comprise a capacitor and a resistor connectedin series across the voltage source, wherein the electrode is connectedto a node between the voltage source and a first terminal of thecapacitor, and the first output signal is taken from a node between asecond terminal of the capacitor and the resistor.

The electrode may have a surface from which the electric field extends,at least a portion of that surface which in use will be closest to therotating member being covered by body of protective material. Theprotective material may for instance be non-conductive.

The electrode may comprise an electrode body and an electrode surfacefrom which the electric field extends, at least a portion of thatsurface which in use will be closest to the rotating member beingdefined by a conductive material different from the body of theelectrode.

According to a second aspect of the present invention there is provideda rotating machine comprising:

-   -   a rotationally salient rotating member supported for rotation        about an axis and a speed sensor for sensing the rotational        speed of the rotating member, the speed sensor comprising:    -   an electrode fixed in position relative to said axis;    -   a constant voltage source connected to the electrode for        establishing an electric field between the electrode and a        substantially constant potential portion of the rotating        machine,    -   wherein the electrode is positioned so that at least one salient        feature of the rotating member passes through the electric field        as the rotating member rotates about said axis, thereby        perturbing the electric field on each revolution of the rotating        member;    -   a current detector for detecting current flow between the        voltage source and the electrode in response to said        perturbation of the electric field; and    -   an amplifier circuit comprising a signal amplifier for        amplifying the first signal and outputting a second signal        modulated at a frequency corresponding to the frequency of        perturbation of the electric field;    -   wherein the electrode voltage, the amplifier gain and the        electrode position relative to the rotating member are selected        such that modulation of the second signal is predominantly        caused by perturbation of the electric field within the        dielectric medium by the creation and/or movement of ions within        the field.

The substantially constant potential portion of the rotating machine maybe said rotating member.

The rotating member may for instance be non-conductive and said constantpotential portion of the rotating machine may be a conductive bodysupported by the rotating machine in a fixed spatial relationship to theelectrode so that said at least one salient feature of the rotatingmember passes between the electrode and the constant potential portionon each revolution of the rotating member about said axis.

The constant potential portion of the rotating machine may for instancebe a conductive portion of a housing of the rotating machine.Alternatively, the rotating member may be mounted on a shaft at least aportion of which is conductive and comprises said constant potentialportion of the rotating machine.

The rotating member may comprise a plurality of said salient features.Preferably a surface of said electrode though which the electric fieldpasses has an extent in the rotational direction of the rotating memberless than the separation of rotationally adjacent salient features ofthe rotating member.

The or each salient feature may comprises a physical protuberance and/orlocalised change in material properties of the rotating member.

In some embodiments the salient feature is a physical protuberanceincluding an edge.

The or each salient feature may be a protuberance extending generallyradially and/or axially relative to said axis. For instance, the or eachsalient feature may a blade or fin.

The rotating member may rotate within a chamber defined by a housing, afluid being present within said chamber, such that the electric fieldpasses through said fluid between the electrode and the rotating member.The housing may have a fluid inlet and a fluid outlet, and said chambermay lie in a fluid flow path between said inlet and said outlet.

The rotating machine may be a turbomachine. For example, the rotatingmachine may be a turbine or a compressor (included for instance in aturbocharger or other) and said rotating member may be a turbine wheelor a compressor wheel.

The electrode may be supported by a member inserted into the inlet oroutlet and electrically insulated from said member. The inserted membermay be an annular member such as for instance a compressor noise baffle.

In some embodiments the electrode is supported by a housing of therotating machine and is electrically insulated from said housing.

The electrode is preferably not exposed to said fluid. The electrode mayfor instance be separated from said fluid by a layer or body of materialwhich is at least substantially non-reactive to said fluid. Theelectrode may be separated from said fluid by a layer or body ofelectrically insulating material.

Irrespective of the method of supporting the electrode, in someembodiments (such as for example where the rotating machine is acompressor) it preferably has a fixed location with respect to ahousing, and a set minimum distance from the rotating member (thisminimum may only be at one or more orientations of the wheel, and may bedependent upon the wheel taking up any axial play in a bearing, and anyordinary thermal and centripetal expansion). This minimum distance ispreferably at least 0.1 mm, more preferably at least 0.2 mm, morepreferably at least 0.3 mm, more preferably at least 0.4 mm, morepreferably at least 0.6 mm, more preferably at least 0.8 mm, morepreferably at least 1 mm, more preferably at least 1.5 mm, morepreferably at least 2 mm, more preferably at least 5 mm, and mostpreferably at least 10 mm. This distance is also preferably less thanthree times the diameter of the rotary member, or a part thereof passingthrough the electric field area, and more preferably less than saiddiameter, and most preferably less than a third thereof. Where therotating member has an array of blades, the distance is furtherpreferably less than the separation of the blades in the electric fieldarea, more preferably half thereof.

The constant potential portion of the machine may be at ground potentialor a virtual ground potential.

The speed sensor may be a speed sensor according to any other aspect ofthe invention.

In a third aspect of the invention there is provided a method ofmeasuring the speed of a rotating machine comprising a rotationallysalient rotating member supported for rotation about an axis using aspeed sensor according to any other aspect of the invention.

According to a fourth aspect of the invention there is provided a methodof measuring the speed of a rotating machine comprising a rotationallysalient rotating member supported for rotation about an axis, the methodcomprising:

-   -   supporting an electrode in a position relative to said axis;    -   supplying a constant voltage to the electrode to establish an        electric field between the electrode and a substantially        constant potential portion of the rotating machine;    -   wherein the electrode is positioned so that at least one salient        feature of the rotating member passes through the electric field        as the rotating member rotates about said axis, thereby        perturbing the electric field on each revolution of the rotating        member;    -   detecting current flow between the voltage source and the        electrode in response to said perturbation of the electric field        to derive a first signal modulated at the frequency of said        perturbation;    -   amplifying the first signal and outputting a second signal        modulated at a frequency corresponding to the frequency of        perturbation of the electric field;    -   wherein the electrode voltage, the amplifier gain and the        electrode position relative to the rotating member are selected        such that modulation of the second signal is predominantly        caused by perturbation of the electric field within the        dielectric medium by the creation and/or movement of ions within        the field; and determining the rotational speed from the        modulation of said signal.

According to a fifth aspect the invention provides a speed sensor foruse in measuring the speed of rotation of a rotationally salientrotating member, the speed sensor comprising an electrode and a sensorcircuit;

the sensor circuit comprising:

a constant voltage source for supplying a voltage to the electrode togenerate an electric field;

a current detector for detecting current flow between the constantvoltage source and the electrode due to perturbation of the electricfield by passage of at least one salient feature of the rotating memberthrough the electric field as the rotating member rotates;

the current detector outputting a first output signal modulated at afrequency corresponding to the frequency of perturbation of the electricfield.

The sensor circuit preferably further comprises a signal amplifier foramplifying the first output signal and outputting a second outputsignal.

The electrode voltage, amplifier gain and distance between the electrodeand the rotating member may be selected such that pulses in theelectrode current would be generated predominantly by the movement ofions passing through the localised electric field. The ions may be atleast one of: within a fluid or adsorbed on a non-conducting solid.

At least a portion of the speed sensor may be electrically shielded byat least one conductive shield which may be held at a controlledelectric potential. Preferably the sensor includes at least one shieldamplifier the output of which drives the or each conductive shield. Theor each shield amplifier may be coupled to the signal amplifier. Ashield amplifier may for instance be connected in series with the signalamplifier, and the sensor circuit may include a feed back loop connectedacross the signal amplifier and said shield amplifier, the feedback loopincluding a first capacitor. The first capacitor may be part ofcapacitive bridge (which may comprises the first capacitor and atpotential divider) which is adapted to reduce the effective capacitanceof the capacitor.

According to a sixth aspect the invention provides a method of measuringthe speed of a rotating machine comprising a rotationally salientrotating member supported for rotation about an axis, the methodcomprising:

supporting an electrode in a position relative to said axis;

supplying a constant voltage to the electrode to establish an electricfield between the electrode and a substantially constant potentialportion of the rotating machine;

wherein the electrode is positioned so that at least one salient featureof the rotating member passes through the electric field as the rotatingmember rotates about said axis, thereby perturbing the electric field oneach revolution of the rotating member;

detecting current flow between the voltage source and the electrode inresponse to said perturbation of the electric field to derive a signalmodulated at the frequency of said perturbation; and

determining the rotational speed from the modulation of said signal.

It is believed that the sensor operates by detecting perturbation in theelectric field due or more of the following:

a modulation of charge distribution within the electric field due to themovement of charge generated by friction (triboelectric effect). Thefriction may for instance be within the fluid itself (such asturbulence) or between the fluid and a body within the fluid conduit(such as a salient feature of the rotating member);

a modulation of charge distribution within the electric field due to avariation in physical properties of the fluid caused by movement of asalient feature of the rotating member through the electric field (e.g.increased pressure leading to increased charge density);

a modulation of the electric properties of any dielectric materialwithin which the electric field is created (e.g. pressure and/ortemperature and/or humidity or other changes affecting the dielectricconstant of the dielectric material).

Accordingly one aspect of the present invention is a method of measuringthe rotational speed of a rotationally salient member supported forrotation about an axis, the method comprising:

supporting an electrode in a position relative to said axis;

supplying a constant voltage to the electrode to establish an electricfield between the electrode and a substantially constant potential body;

wherein the electrode is positioned so that at least one salient featureof the rotating member passes through the electric field as the rotatingmember rotates about said axis, thereby perturbing the electric field oneach revolution of the rotating member;

detecting current flow between the voltage source and the electrode inresponse to perturbation of the electric field to derive a signalmodulated at the frequency of said perturbation; and

determining the rotational speed from the modulation of said signal;

wherein said perturbation is due to one or more of the followingeffects:

-   -   a modulation of charge distribution within the electric field        due to the movement of charge generated by friction        (triboelectric effect). The friction may for instance be within        the fluid itself (such as turbulence) or between the fluid and a        body within the fluid conduit (such as a salient feature of the        rotating member);

a modulation of charge distribution within the electric field due to avariation in physical properties of the fluid caused by movement of asalient feature of the rotating member through the electric field (e.g.increased pressure leading to increased charge density); and

a modulation of the electric properties of any dielectric materialwithin which the electric field is created (e.g. pressure and/ortemperature and/or humidity or other changes affecting the dielectricconstant of the dielectric material).

Similarly the present invention provides a sensor for use in measuringthe rotational speed of a rotationally salient member supported forrotation about an axis, the sensor comprising an electrode and a sensorcircuit;

the sensor circuit comprising:

a constant voltage source for supplying a voltage to the electrode togenerate an electric field;

a current detector for detecting current flow between the constantvoltage source and the electrode due to perturbation of the electricfield by passage of at least one salient feature of the rotating memberthrough the electric field as the rotating member rotates;

the current detector outputting an output signal modulated at afrequency corresponding to the frequency of perturbation of the electricfield;

wherein the perturbation of the electric field is caused by one or moreof the following effects:

-   -   a modulation of charge distribution within the electric field        due to the movement of charge generated by friction        (triboelectric effect). The friction may for instance be within        the fluid itself (such as turbulence) or between the fluid and a        body within the fluid conduit (such as a salient feature of the        rotating member);

a modulation of charge distribution within the electric field due to avariation in physical properties of the fluid caused by movement of asalient feature of the rotating member through the electric field (e.g.increased pressure leading to increased charge density); and

a modulation of the electric properties of any dielectric materialwithin which the electric field is created (e.g. pressure and/ortemperature and/or humidity or other changes affecting the dielectricconstant of the dielectric material).

The sensor circuit preferably includes an amplifier the operatingcharacteristics of which are adapted so as to amplify the output signaldue to one or more of these effects whilst minimising signal noise whichmay otherwise impede detection of signal modulation due to saidperturbation. The amplifier could for instance include a signalamplifier and shield amplifier as discussed above.

The invention also contemplates the provision of a method of measuringchanges in a characteristic of a fluid, the method comprising:

supporting an electrode in a position relative to the fluid;

supplying a constant voltage to the electrode to establish an electricfield between the electrode and a substantially constant potential body,the electrode being positioned so that the electric field extends intosaid fluid:

detecting current flow between the voltage source and the electrode inresponse to perturbation of the electric field due to changes in acharacteristic of the fluid; and determining the change in saidcharacteristic from the detected current flow;

wherein said perturbation is due to:

a modulation of charge distribution within the electric field due to themovement of charge caused by a change in said characteristic; and/or

a modulation of the dielectric constant of said fluid due to change insaid characteristic.

The characteristic may for instance be temperature, pressure, humidityor chemical composition of the fluid.

The invention also contemplates the provision of a method for measuringmass flow of fluid flowing through a conduit, the method comprising:

supporting an electrode in a first position relative to said conduit;

supplying a constant voltage to the electrode to establish an electricfield between the electrode and a substantially constant potential body,the electrode being positioned so that the electric field extends intosaid conduit;

modifying a characteristic of the fluid at a second position upstream ofthe first position;

detecting current flow between the voltage source and the electrode inresponse to perturbation of the electric field due to said modifiedcharacteristic of the fluid; and

determining the mass flow of the fluid from the separation of said firstand second positions and the time lapse between said modification of thefluid characteristic and said current detection.

Other preferred and particularly advantageous features of the inventionwill be apparent from the following description.

Specific embodiments of the present invention will now be described, byway of example only, with reference to the accompanying drawings, inwhich:

FIG. 1 is an axial cross-section through a turbocharger with a fixedgeometry turbine which illustrates the basic components of aturbocharger;

FIG. 2 is a perspective view of a turbocharger compressor housing inaccordance with a first embodiment of the invention;

FIG. 3 is a cross-section through a portion of the compressor of FIG. 2;

FIG. 4 shows a longitudinal cross-section through a sensor electrodeassembly of the compressor of FIGS. 2 and 3;

FIG. 4a shows schematic features of a sensor circuit in accordance withthe invention;

FIG. 5 is a perspective view of a turbocharger compressor housing inaccordance with a second embodiment of the invention;

FIG. 6 is a cross-sectional view of a noise baffle insert of theturbocharger compressor of FIG. 5;

FIG. 7 cross-section through a portion of the compressor of FIG. 5;

FIG. 8 is a schematic circuit diagram of an embodiment of a sensorcircuit in accordance with the present invention;

FIG. 9 is a circuit diagram of a further embodiment of a sensor circuitaccording to the present invention;

FIG. 10a is a simplified diagrammatic cross-section through anelectrically conductive compressor wheel and adjacent electrode assemblyin accordance with the present invention;

FIG. 10b is a simplified diagrammatic cross-section through anelectrically non-conductive compressor wheel supported on anelectrically conductive shaft and adjacent electrode assembly inaccordance with the present invention;

FIG. 10c is a simplified diagrammatic cross-section through anelectrically non-conductive compressor wheel and adjacent electrodeassembly in accordance with the present invention; and

FIGS. 11a and 11b schematically illustrate electric field lines betweenan electrode of a sensor according to the invention and a rotatingmember.

Referring first to FIG. 1, this is an axial cross-section through atypical turbocharger with a fixed geometry turbine which illustrates thebasic components of a turbocharger. The turbocharger comprises a turbine1 joined to a compressor 2 via a central bearing housing 3. The turbine1 comprises a turbine housing 4 which houses a turbine wheel 5.Similarly, the compressor 2 comprises a compressor housing 6 whichhouses a compressor wheel 7. The turbine wheel 5 and compressor wheel 7are mounted on opposite ends of a common turbo shaft 8 which issupported on bearing assemblies 9 within the bearing housing 3.

The turbine housing 4 is provided with an exhaust gas inlet 10 and anexhaust gas outlet 11. The inlet 10 directs incoming exhaust gas to anannular inlet chamber, i.e. volute 12, surrounding the turbine wheel 5and communicating therewith via a radially extending annular inletpassageway 13. Rotation of the turbine wheel 5 rotates the compressorwheel 7 which draws in air through an axial inlet 14, defined in part byan annular inlet wall 6 a, and delivers compressed air to the engineintake (not shown) via an outlet 15.

FIGS. 2 to 4 a illustrate a turbocharger compressor including aturbocharger speed sensor in accordance with a first embodiment of theinvention. Although features of the compressor housing 6 differ indetail from those of the turbocharger compressor of FIG. 1, theconstruction is generally the same and where appropriate the samereference numbers are used to identify corresponding features. A speedsensor assembly 16 according to the invention is fitted to thecompressor housing 6. The sensor assembly 16 (shown in isolation in FIG.4) comprises an elongate electrode 17 supported within an electricallyconductive guard tube 18 which extends through a generally radial bore19 provided through the compressor housing wall. The bore 19 opens at aninternal surface 20 of the compressor which is swept by the blades ofthe compressor wheel 7 so that an enlarged end 21 of the electrode 17 islocated in close proximity to the compressor wheel 7. The opposite endof the electrode 17, and the guard tube 18, are connected to a PCB 22which is housed within an electrically insulating sensor casing 23,which includes a tubular portion 24 which sheaths the guard tube 18. Thesensor assembly 16 is secured to the housing 6 by a bolt 25 which passesthrough an aperture 26 in the sensor casing 23 and into a threaded bore27 in the housing wall. An o-ring seal 28 is provided to seal sensorwithin the bore 19. An additional threaded bore 28 a is also provided soas to facilitate the location of an inlet sensor device, such as atemperature sensor, (not shown).

The PCB 22 electrically connects the electrode 17 and the guard tube 18to a sensor circuit 29 located on the PCB 22 (shown schematically inFIG. 4a ). The sensor circuit 29 comprises a fixed voltage source 31, acurrent detection circuit 32, an amplifier 33 and a frequency counter34. The frequency counter may comprise a frequency divider (not shown).The frequency counter outputs a signal representative of the rotationalspeed of the compressor wheel which may be for instance be supplied, viaconnecting cable 30, to an engine ECU or other controller. Furtherdetails, and operation, of exemplary embodiments of the sensor circuitare described later.

A second embodiment of a turbocharger speed sensor according to thepresent invention is illustrated in FIGS. 5 to 7. Again, althoughfeatures of the compressor housing differ in detail from those of theturbocharger compressor of FIGS. 1 and 2 to 4, where appropriate thesame reference numbers are used to identify corresponding features.Referring first to FIG. 5, the compressor includes a noise baffle 35 inthe form plastic insert which is installed in the compressor inlet 14.Provision of a compressor noise baffle as an insert separate to thecompressor housing is known in the art and the function of the noisebaffle will not be described in detail.

However, as can be seen in FIG. 6 the noise baffle 35 is an annularstructure which includes a tubular portion 36.

In accordance with the present invention the compressor includes asensor assembly 16 which includes an electrode 17 which is embedded inthe plastic noise baffle insert 35 as shown in FIGS. 6 and 7. Theelectrode 17 is generally L-shaped having an axially extending portion17 a and a radially extending portion 17 b which projects into a bore 37provided in the baffle insert 35 for connection to other components ofthe sensor assembly 16 as illustrated in FIG. 7.

Referring to FIG. 7, the compressor housing 6 is provided with a radialconnector aperture 38 in the inlet wall 6 a which aligns with the bore37 of the baffle insert 35 to facilitate electrical connection to theradial portion 17 b of the electrode 17. The tubular portion 36 of theinsert 35 extends towards the compressor wheel (not shown) so that theend of the axial portion 17 a of the electrode 17 lies in closeproximity to the compressor wheel.

The sensor assembly 16 further comprises a electrically insulatingsensor casing 23 which houses a PCB 22 connected to an electricallyconductive socket 39. The socket 39 is located in a tubular portion 40of the casing 23 which is inserted into the baffle bore so that theradial portion b of the electrode porting extends into the socket 31.The tubular portion 40 may be an interference fit in the bore 37 and theelectrode portion 17 b may be an interference fit in the socket 39. Theend of the electrode portion 17 b is tapered to aid the insertion intothe socket 39. An O-ring seal 28 seals the sensor casing 23 with respectto the aperture 38. The sensor assembly 16 is secured to the housing 6by a bolt 25 which extends through an aperture 26 in the casing 23 andinto a threaded bore 27 in the housing wall 69.

The PCB 22 connects the electrode 17 (via the sensor circuit 29 situatedon the PCB 22) to an output pin 41 which is provided for connection toan engine ECU or other controller. The sensor circuit is not shown, butis the same as the sensor circuit 29 illustrated schematically in FIG. 4a.

Embedding the electrode 17 within a removable insert 35 ensures thatintegrating the sensor into the turbocharger is simple and can be doneat very low cost. The electrode 17 may for instance be a simpleconductive (e.g. copper) wire. Preferably it is of a durable metal.Different compressor inlet configurations can be readily be accommodatedby appropriately configuring the baffle insert 35, without the need tochange other details of the sensor assembly 16 or compressor housing 6.The electrode can also be replaced if necessary simply by replacing theremovable baffle insert 35.

Both above embodiments of the invention have the same sensor circuit 29and operate in the same way. This will now be described.

A voltage V is applied between ground and the electrode 17 by theconstant voltage source 42. This establishes an electric field betweenthe electrode 17 and the compressor wheel 7 which is a virtual ground.As the compressor wheel 7 rotates, the wheel blades successively sweeppast the location of the electrode 17. The electric field between theelectrode and the compressor wheel is perturbed as each blade sweepsthrough the electric field. Current flows between the voltage source andthe electrode in response to the perturbation of the electric field asthe voltage source seeks to maintain voltage V at the electrode. Thecurrent flow is thus modulated with a frequency directly dependant onthe speed of rotation of the compressor wheel. The detected current isamplified by amplifier 33 and the amplified signal is fed to thefrequency counter 34 which outputs a signal representative of theinstantaneous compressor wheel (i.e. turbocharger) speed.

Exemplary embodiments of the sensor circuit will now be described withreference to FIGS. 8 and 9.

Referring first to FIG. 8, this shows a schematic circuit illustratingthe operation principles of a portion of the sensor circuit 29. A DCpower supply 42 is electrically connected via a resistor 43 to theelectrode 17. The power supply 42 is also electrically connected to acommon ground 44. The common ground 44 may be a conductive component ormay be bodywork or chassis of a vehicle in which the turbocharger islocated, or may be earth (particular in the case of a turbochargerfitted to a stationary engine such as a power generator). The compressorwheel is also at the common ground potential indicated by 45. Thisestablishes an electric field between the electrode and the compressorwheel. As discussed above, a modulated current will flow between thevoltage source and the electrode as the compressor wheel rotates. Thiscurrent will cause the voltage at a junction 46 intermediate theelectrode 17 and resistor 43 to change. The junction 46 is electricallyconnected to the common ground 44 via a capacitor 47 and resistor 48.The change in voltage at junction 46 will cause a current to flowthrough the coupling capacitor 47, which will result in a modulation ofthe voltage at a second junction 49 intermediate the capacitor 47 andthe resistor 48. The modulation of voltage at junction 49 is amplifiedby the connected amplifier 33, the output 50 of which is then suppliedto the frequency counter 34. The frequency counter (not shown in FIG. 8)may be entirely conventional.

FIG. 9 illustrates a second embodiment of a portion of the sensorcircuit 29. Due to the fact that the electrode 17 and its electricalconnection to the sensor circuit is very sensitive to adverse electricfields external to the turbocharger, for example radio interference, theelectrode 17 and the electrical connection between the electrode and thesensor circuit 29 is screened. In the case of the electrode of the abovedescribed embodiments of the invention, the adverse external electricfield screening is conveniently provided by the metal compressor housing6, and in the case of the embodiment of FIGS. 2 to 4 screening is alsoprovided by the guard tube 18. As such, if the electrode 17 is to belocated remotely to at least part the detection circuitry (especiallythe amplifier), it is important that screened cable, such as co-axialcable, is used to connect the electrode 17 to the detection circuitry.The inner core 51 of the screened cable carries the signal from theelectrode 17, and is surrounded by an outer shield layer 52. Theco-axial cable acts as a capacitor. Extraneous capacitance in the cableis compensated for by a guard amplifier 53. The guard amplifier is heldat a controlled potential, for example, but not limited to, the virtualground potential.

As with the previous circuitry embodiment, a DC power supply 42 iselectrically connected to the electrode 17. Intermediate the powersupply 42 and electrode 17 is a bias circuit 54 comprising resistors 55,56 and 57 and capacitor 58. The purpose of the bias circuit is toincrease the output signal to noise ratio. The capacitor 58 of the biascircuit 54 is electrically linked to the output of the guard amplifier53 such that the bias circuit 54 provides a larger impedance to theelectrode 17. The function of the capacitor 47 in this embodiment isvery similar to that of the like numbered capacitor in the previousembodiment, in that it connects the electrode 17 to the input of theguard amplifier 53. The guard amplifier 53 functions so as to nullifyany difference in potential between an amplifier ground and a compressorwheel ground. Unlike in the first circuitry embodiment, the compressorwheel ground and amplifier may not be connected to a common ground. Anydifference between the amplifier ground and compressor wheel ground willresult in inaccurate measurements. The output of the guard amplifier 53feeds the outer shield layer 52 of the co-axial cable. This maintains aconstant voltage at the amplifier, such that only changes in theelectrode 17 signal are amplified. In this way, unwanted signal content,i.e. noise is removed.

The output from the guard amplifier 53 is fed via an intermediatecapacitor 59 to the input of an inverting amplifier 60. A capacitivedivider 61, comprising resistors 62, 63 and 64 and capacitor 65, iselectrically connected around the amplifier 60 and acts as a feedbackelement which biases the amplifier 60. In addition, the capacitivedivider 61, in combination with the capacitor 59, constitutes a highpass filter. The resistor 62 has a very high effective resistance sothat the high pass filter has a low cut-off frequency so that the sensorhas an overall bandwidth that starts at a low frequency. A low passfilter (not shown) may also be used to remove high frequency componentsof the signal which are not related to the motion of the compressorwheel.

The use of two amplifiers in series, in this case the guard amplifier 53and inverting amplifier 60 is advantageous in that it offers a costsaving over the use of just one high quality amplifier which may becapable of performing a similar function.

A further capacitive divider 66, comprising capacitors 67 and 68 andresistors 69 and 70, is electrically linked between the input of theguard amplifier 53 and the output of the inverting amplifier 60. Thecapacitive divider 66 acts as a feedback element around both amplifiers53, 60. The capacitive divider 66 provides an effective small feedbackcapacitor 67, due to the potential divider action of resistors 69 and70, resistor 69 being indirectly electrically linked and referenced tothe guard amplifier 53 output. The circuit output can be taken at point71, from which it may then be supplied to the frequency counter (notshown) if necessary after further amplification.

The gain of the amplifiers 53, 60 is controlled by the capacitivedivider 66. The capacitive divider preferably provides an effectivecapacitance around the amplifiers 53, 60 which is preferably less than10 pF, more preferably less than 1 pF, more preferably less than 0.1 pF,more preferably less than 0.01 pF, and further preferably still lessthan 0.001 pF. Alternatively, the capacitive divider 66 may be replacedby a single capacitor connected around the amplifiers 53, 60. In thiscase the capacitor preferably has a value less than 100 pF, morepreferably less than 10 pF, more preferably less than 1 pF, morepreferably less than 0.1 pF, and further preferably still less than 0.01pF.

The use of a capacitive divider 66 (or equivalent capacitor) with avalue in the range discussed above controls the gain of the amplifiers53, 60 such that a utilised electrode 17 can have a smaller effectivearea and/or be placed a greater distance away from the compressor wheel,and detect a perturbation of the electric field which would not bedetected by a similar sensor with larger conventional capacitors. Thisis particularly advantageous if the sensor is utilised to measure thespeed of a large compressor, where factors such as axial movement andthermal expansion of the compressor wheel may limit the proximity of theelectrode 17 to the compressor wheel 7.

With each of the embodiments described above, the rotational speed ofthe compressor wheel (i.e. turbocharger) may be determined simply bydividing the modulation frequency by the number of blades on thecompressor wheel 7. This calculation may be performed within thefrequency counter 34 or for instance by an ECU or other controller (suchas for instance an actuator of a turbocharger wastegate or variablegeometry inlet mechanism). The output from the frequency counter 34 mayfor example be in CAN (Controller Area Network) format, as will beappreciated by those skilled in the art. Providing the ECU withinformation on the turbocharger speed may for instance allow the ECU tooptimise critical operational parameters of the engine to which theturbocharger is attached and also allow remedial action to prevent anypossible damage to the turbocharger and/or engine in the event of theturbocharger over speeding.

Since known capacitive sensors rely on the modulation of the couplingbetween the electrode and the compressor wheel to produce a change incapacitance, they will not function if the compressor wheel is made ofan electrically non-conducting material. This is because, without aconducting compressor wheel, there is no modulation of the capacitanceof the sensing circuit related to the rotation of the turbomachine. Onthe contrary, as mentioned above, it has been shown that the sensor ofthe proposed invention will operate with an electrically non-conductingcompressor wheel, for example, one made of a plastics material. This isbecause a plastics compressor wheel will still produce a detectableperturbation in the electric field.

Another problem the current invention overcomes is that of an unstableground potential. One side of the power supply 42 is connected to acommon ground, such as for instance the turbocharger housing, the engineblock to which the turbocharger is attached and the bodywork/chassis ofthe vehicle of which the turbocharger forms part. As such, the groundpotential of the turbocharger housing must be conducted to thecompressor wheel 7. However, the compressor wheel 7 is typicallysupported, via the shaft 8 on oil fed journal bearings 9. Thus at leastone oil film exists between the compressor wheel 7 and the groundpotential. As the shaft 8 rotates, the thickness of the at least onefilm may alter, which in combination with the resistively of the oilchanging due to soot content, may lead to an unstable difference betweenthe ground potential of the turbocharger housing and that of thecompressor wheel 7. An unstable ground potential may lead to erroneoussignals at the electrode 17. The guard amplifier 33 substantiallyeliminates the adverse effects of unstable ground potential, as changesin potential between amplifier ground and the compressor wheel 7 arenullified by applying feedback in order to maintain a constant voltageat the amplifier 33.

It will be appreciated that many modifications can be made to the detailof the embodiments of the invention described above. For instance, theguard tube 18 of the first embodiment can be omitted if the shielding isprovided by the compressor housing 6. However, in other embodiments ofthe invention, the housing 6 may not be made of a suitably conductivematerial in which case the guard tube 18 will be useful. Similarly,whereas the second embodiment described above has no dedicated electricfield shield, as this is provided by the compressor housing, such ashield (similar to the guard tube 18) could be added if necessary.

In the second embodiment of the invention described above the electrode17 may be such that its axial portion 17 a is order of 100 mm in length.The surface area of the electrode 17 in either of the above embodimentsmay typically be between 1 mm² and 50 mm². In other embodiments the sizeof the electrode may vary. For instance, the electrode 17 may be ribbonshaped or may have an axial portion 17 a comprising an enlarged spadeportion (for instance it may end in a ‘T’ shape). Additionally theenlargement may be in two dimensions to form a base (e.g. with a drawingpin or thumbtack shape) which may be shaped substantially as a rectangleor trapezoid, or to match a portion of the surface swept by thecompressor wheel blades (or the rotationally symmetric member). e.g. asan elongated trapezoid being curved in both its axes.

Although the electrode 17 may be of any appropriate size or shape, it ispreferable that the angular extent of the electrode 17 with respect tothe axis of rotation of the compressor wheel is less than half theangular separation between adjacent blades on the compressor wheel. Thisminimises time averaging of the perturbation of the electric fieldcaused by the rotation of the compressor wheel and hence reducessmoothing of the signal.

For instance, it will be appreciated that the physical structure of theelectrode 17 can vary. A simple conductive wire will be sufficient inmany applications, in others a different configuration might beadvantageous to establish the required electric field. In applicationswhere the electrode 17 might be exposed to contamination it ispreferable that the electrode surface is not exposed, as with theembodiments described above, unless the electrode is constructed ofmaterial which is not chemically reactive in particular environment. Itis however useful to have the electrode separated from the fluid (e.g.gas) flow by a material which has a high dielectric constant. Similarlythe configuration and construction of the sensor casing 23 may varydependant upon the application and manner in which the sensor is to besupported and connected.

Although within the second embodiment of the invention described above,the electrode 17 is embedded within a noise baffle 35, it will beappreciated that the sensor may be embedded within any appropriateinsert which is shaped so as to be received by the inlet 14.

The location of the sensor may also vary from that illustrated. It isadvantageous to locate the electrode in close proximity to thecompressor wheel (or other rotating component being monitored) where theperturbation of the electric field is greatest. It may also bepreferably that the electrode is not located so as to have anysignificant adverse effect on efficient gas flow through the compressor.However, the electrode may be located in any position at which there issufficient perturbation of the electric field for accurate detection.This may be either upstream or downstream of the compressor wheel withrespect to the direction of gas flow and the electrode could forinstance be located in either the inlet 14 or outlet 15. This could forinstance be determined by simple testing in any particular applicationof the invention. In addition, the sensor circuit 29 could be locatedremote from the electrode 19.

In the above-described embodiments, the sensor circuit 29 is situated onthe PCB 22. This need not be the case. The sensor circuit comprises afixed voltage source 31, a current detection circuit 32, an amplifier 33and a frequency counter 34. The frequency counter may comprise afrequency divider. Any of these circuit components may be locatedremotely to the PCB 22 and/or sensor assembly 16. For instance, thefixed voltage source 31 and frequency counter 34 may form part of an ECUor other controller, whereas the current detection circuit 32 andamplifier may be located on the PCB 22. Of course, electricalconnections will always be required between the electrode 17 andrespective components of the detection circuit 32. In another perceivedembodiment, the frequency divider may be located on the PCB 22, itsoutput being provided to a remote frequency counter which is connectedto an ECU or other controller.

As such the sensor components may be split into two or more groups oneof which may be physically positioned proximal to the electrode, and oneof which may be physically positioned remotely such as adjacent to orwithin an ECU. It is however preferable that the amplifier is close tothe electrode to minimise the likelihood of electrical interference. Ifthe amplifier is located remotely from the electrode the connectionbetween the two is preferably electrically shielded.

Alternative embodiments of sensor circuit adapted to measure themodulated current flow to the electrode could be readily devised by theappropriately skilled person. The voltage applied between the electrodeand the compressor wheel (or other rotating member) may be determined asnecessary to establish an electric field which exhibits a measurableperturbation. In some applications a voltage greater than about 30V willbe appropriate, but for many applications the voltage is preferablygreater than 50V, and more preferably greater than 70V. Voltages of over120V may be used as this is readily accommodated by readily availableelectronic components. Much higher voltages, e.g. above 500V or above 1kV, may well be advantageous in some applications. However, the use ofsuch high voltages may necessitate the use of more robust circuitrycomponents. The voltage at the electrode may be positive or negative inrelation to compressor wheel (or other rotating member) so that theelectrode is either an anode or a cathode with respect to the compressorwheel or other rotating member (which may be electrically conductive ornon-conductive).

According to one embodiment the sensor circuit may comprise just onehigh gain amplifier, although for some applications this may require anexpensive high quality amplifier. According to another embodiment thereare two amplifiers, one being a guard (or shield) amplifier. The lattercase offers the opportunity to use cheaper, and lower qualitycomponents. Although the guard amplifier may operate at any appropriategain, it is preferable that it operates with a gain of substantially 1,so as to hold the outer shield layer of the screened cable and externalelectric field screen at the same potential.

In the above described embodiments of the invention, the electrode ismounted to the compressor housing to measure rotational speed of thecompressor wheel. In other embodiments the electrode could be arrangedto directly measure rotation of the turbine wheel or of some otherfeature of the turbocharger shaft. As such the sensor could be mountedto the turbine or bearing housings.

It will be appreciated that the invention is not limited in applicationto turbochargers but can be used to measure the rotational speed ofother rotating machines. Examples of such rotating machines include, butare not limited to, a gas or liquid turbine (including for instance apower turbine or gas turbine or other form of engine), or a gas/liquidcompressor which is not part of a turbocharger, or for instance a fluidpump. In such alternate applications the apparatus housing and/or therotating member being monitored may be non-conductive, such as forinstance a plastics impeller wheel rotating in a plastics housing.

It will also be appreciated that the choice of material of which thesensor casing, apparatus housing or rotating member is made may be suchas to increase the perturbation of the electric field. This may dependon a property of the material, such as its place within thetriboelectric series.

Furthermore, it will be appreciated that if the invention is to beapplied to a rotating machine that does not comprise a rotationallysalient member, then a rotationally salient member must be mechanicallylinked thereto. For instance, the speed of a shaft, or other rotatingbody which has no rotationally salient feature, may be measured bymechanically linking a member with a rotationally salient feature to theshaft (or other rotating body) such that they co-rotate. Accordingly, asensor according to the present invention may detect the rotation of theco-rotating member and hence determine the speed of rotation of theshaft (or other rotating body).

As described above the invention measures perturbation in the electricfield established between the electrode and the rotating member (e.g.compressor wheel). Without wishing to be bound by theory, it is believedthat the perturbation may result from several effects discussed below.

FIG. 10a schematically illustrates an example of an electrode 72embedded in a body 73 in proximity to a rotating member, in thisillustration an electrically conductive compressor wheel 74. The body 73may be fabricated from a non-conductive material (which may for instancebe ceramic or a plastics member such as the baffle insert 35 describedabove or sensor casing 23) in which case it may electrically (andthermally) insulate the electrode 72 from a supporting housing or othersupport structure. Alternatively the body 73 could be fabricated from aconductive material, and if necessary electrically insulated from asurround housing or support structure (unless the support is itselfnon-conductive). In the latter case the body would preferably comprise aconductive material which is unreactive in the environment to which itis exposed (e.g. platinum). The body could also be omitted entirely, inwhich case it would be preferably for the electrode to be non-reactive.

Application of a voltage as described above establishes an electricfield between the electrode 72 and a constant potential portion, whichin this case is the wheel 74. Both the body 73 and the gas 75 (i.e. air)flowing between the wheel 74 and the body 73 behave as dielectricmaterials with different dielectric properties. The electric fieldextends a total distance A between the electrode 72 and the wheel 74. Ofthe total distance A, a portion B of that distance is through gas 75(air in this case) between the wheel 74 and the body 73. The electricfield extends a portion C of the distance A through the body 73.

FIG. 10b schematically illustrates another example of an electrode 72embedded in a body 73 in proximity to a rotating member, in thisillustration an electrically non-conductive compressor wheel 74 a, whichis rotatably mounted on electrically conductive shaft 8. Application ofa voltage as described above, between the electrode 72 and shaft 8,establishes an electric field between the electrode 72 and a constantpotential portion, which in this case is the shaft 8. The body 73, thegas 75 (i.e. air flowing between the wheel 74 a and the body 73) and thenon-conductive compressor wheel 74 a all behave as dielectric materialswith different dielectric properties. The electric field extends a totaldistance A between the electrode 72 and the shaft 8. Of the totaldistance A, a portion B of that distance is through gas 75 (air in thiscase) between the wheel 74 and the body 73. The electric field extends aportion C of the distance A through the body 73. The electric fieldextends a portion D through the compressor wheel.

FIG. 10c schematically illustrates a further example of an electrode 72embedded in a body 73 in proximity to a rotating member, in thisillustration an electrically non-conductive compressor wheel 74 b.Application of a voltage to the electrode 72 as described above,establishes an electric field between the electrode 72 and a constantpotential portion of the compressor 78, which could for instance be aportion of the compressor housing (electrically isolated if necessary)or a dedicated electrode supported by the housing. The body 73, the gas75 (i.e. air flowing between the wheel 74, the body 73 and the constantpotential portion 78) and the non-conductive compressor wheel 74 b allbehave as dielectric materials with different dielectric properties. Theelectric field extends a total distance A between the electrode 72 andthe constant potential portion 78. Of the total distance A, portions Band E of that is through gas 75 (air in this case) between the wheel 74,the body 73 and the constant potential portion 78. The electric fieldextends a portion C of the distance A through the body 73. The electricfield extends a portion D through the compressor wheel.

Ions may be present in the region of the electric field (the term “ions”used to refer to any charged entity which is affected by the electricfield and for the avoidance of doubt includes charged molecules.Depending on whether or not the electrode is held at a higher or lowerpotential than the wheel, one will constitute an anode and the other acathode. Negatively charged ions will be attracted toward the anode andpositively charged ions will be attracted toward the cathode. Free ormobile ions experiencing this force will move towards either theelectrode 72 or compressor wheel 74. The mobile ions may contact theelectrode 17 or compressor wheel 74 and directly transfer charge theretoand/or their redistribution may cause a perturbation of the electricfield within the dielectric materials. It is thought that a perturbationin the electric field will occur, in particular, where a bulkredistribution of ions occurs such that ions of a particular polarityare localised in a particular region of the field.

Mobile ions may be present in both the gas 75 and body 73 depending ontheir composition. As the compressor wheel 74 rotates, frictional forcesbetween it and the gas 75 may cause the surface of the wheel 74 to loseor gain charge, and hence create ions. Frictional forces between the gas75 and body 73 (and any other surface in the gas flow) may cause theexposed surface of the body 73 (or other surface) to lose or gaincharge, again creating ions. In both of these cases it is likely that,not only will ions be created in the exposed surfaces, but also thatcomplementary ions will be created in the gas 75. Furthermore,turbulence within the gas 75 itself may also result in ion creation. Onepossible mechanism for the creation of ions is electron displacement.

If the ions are created in a conductive material, such as metal, thenthe ion charge is free to move (also known as current flow) within theconductive material under the influence of the containment field. If theions are created in an insulator material the ion charge is not free tomove, and as such, the charge accumulates at the surface of the materialwhere the ions are created. In the case where ions are created in thegas 75, the ions constitute localised charge. In the gas 75 it is theions themselves which are free to move, as opposed to the ion charge ina conductive material.

The illustrated compressor wheel 74 comprises a central main body 76,around which a plurality of radially, outwardly extending blades 77 areequi-angularly spaced. As the compressor wheel rotates, the turbulenceof the gas 75 is more pronounced around the edges of the blades 77. Assuch, creation of ions is promoted when the edges of the blades 77 areproximate the electrode 72. This is due not only to the fact that theturbulence of the gas 75 (and hence any frictional forces leading to ioncreation) may be greater in the region near to the electrode 72, butalso that any electric force on a charged particle, due to the electricfield (such as that on an electron which may be stripped off an entityto produce an ion), is greater the closer the charged particle is to theelectrode 72. If for example the compressor wheel 74 is a conductor, forinstance metal, then charge will also naturally accumulate around thesharp edges of each blade 77 even when the wheel is stationary.

The sensor circuitry attached to the electrode 72 not only comprises avoltage source, which operates to supply or receive charge to and fromthe electrode 72, so that a current flows between the voltage source andthe electrode which is detected. The movement of charge within theelectric field, both on to the electrode 72 or compressor wheel 74, orso as to cause a bulk redistribution and localisation of charge, resultsin a perturbation of the electric field within the dielectric materials.

Due to the fact that the sensor circuitry connected to the electrode 72is capable of supplying current, the perturbation of the electric fieldcauses the circuitry to try to maintain the overall charge potential ofthe system and hence counteract the modification of the dielectricfield. To do this, a compensating current flows through the electrode72, to maintain the voltage V at the electrode.

As the compressor wheel 74 rotates, blades 77 repeatedly pass theelectrode 72. Due to the fact that, as explained above, creation of ionsis promoted when the edges of the blades 77 are proximate the electrode72, rotation of the compressor wheel 74 causes modulated perturbation ofthe electric field (which may be referred to as charge modulation). Assuch, the current which flows through the electrode 72 is also modulatedby the rotation of the compressor wheel 74 at a frequency correspondingthe rotational speed of the wheel. The modulated current may beamplified and fed to a frequency counter so as to determine thefrequency of the modulation and hence the rotational speed of the wheel74.

It is thought that the perturbation of the electric field may also beconsidered as resulting from, at least in part, alteration in at leastone of the dielectric properties of the materials within the electricfield, in this case the gas 75 and body 73. An example of such adielectric property is its dielectric constant. The dielectric constantmay be altered by, for instance, a change in temperature or pressure ofthe materials. This differs from known capacitive type sensors, in thatthey are understood to function so as to detect a change in capacitancein the sensor resulting from a change in distance between the electrodeand the blades, whereas the sensor according to the invention utilisesthe change in charge distribution within or at the surface of thedielectric and a corresponding perturbation of the electric field.

As previously discussed, the electric field is thought to not onlypromote the creation of ions, by allowing electrons to overcome thelocalised potentials of their parent atoms or molecules, but in additionit creates a potential within which both free ions and free charge move(e.g. negatively charged ions or electrons towards an anode etc). Inthis regard the electric field may be regarded as a containment field.Furthermore, the electric field helps to overcome the problem of inputsaturation which occurs in known sensor electronics of a type similar tothat of the proposed invention.

In the absence of the electric field, any ions present within theturbine wheel 74, gas 75 and body 73 are free to drift in a randommanner, due to, for example, thermal motion. As the ions drift, they maycollide with other bodies, such as uncharged atoms and molecules. Forthis reason in the absence of an electric field, the generated chargedions would be substantially free to drift, resulting in drift of theelectric field and hence an unstable potential at the electrode. Thepotential at the electrode may drift to such an extent that it exceedsthe magnitude of any signal resulting from the movement of the turbinewheel 74. As such, said signal would be masked and/or distorted by thedrift potential. In extreme cases, for example, if charged particleswere injected into the electric field, the drift voltage may also exceedthe allowable input voltage of the sensing electronics. This is known asinput saturation. Input saturation occurs when the charge accumulated onthe electrode 72 results in a potential, the magnitude of which isoutside the input range of an amplifier which forms part of saidmeasuring means. By establishing the electric field this may beprevented so that modulation of the electrode potential which occurs asa result of charge modulation within the electric field is detectable asthe amplifier will not be overwhelmed by the input saturation potential.The electric filed prevents a high potential developing on the electrodedue to drift which may otherwise be large enough to damage components inthe sensing circuit.

The use of an electric field fed by a voltage supply, which mighttypically be in the range of 50V to 1 kV, means that the movement of anyion is governed mainly by the electric field and not by thermal motion.As such, the amount of drift of the ions and hence in the dielectricfield is greatly reduced. As a result, any electrode potential noise isreduced, thereby increasing the signal to noise ratio. In addition, theelectric field, in minimising ion drift, substantially prevents inputsaturation, thereby avoiding the need to bias the electrode 72, whichwould attenuate the signal.

The use of a voltage supply is also thought to enhance the amplitude ofthe charge modulation. This may result from the voltage supplyincreasing the movement of mobile electrons across the electric field.This action may be directed to compensate or react to changes in thecharge distribution caused by mechanical action of a rotationallysalient rotating member. In this way, a virtual charge balance potentialmay be achieved mid-way between the containment potential.

The axial profile of the electrode 72 may be shaped such that itcorresponds to the axial profile of the blades 77 when they areproximate the electrode 72. As the majority of charge generation isthought to occur at the edges of the blade 77. If the edges of the blade77 are the same shape as the electrode 72 this may increase the couplingbetween the two, so that the charge transfer between the blades 77 andthe electrode 72 is maximised.

It is thought that the perturbation of the electric field may comprisechanges in the electric field as for instance schematically illustratedin FIGS. 11a and 11b . FIG. 11a shows equipotential field lines aroundan electrode 79 held at a raised voltage by a constant voltage sourceadjacent a non-conductive dielectric rotationally salient rotatingmember 80. FIG. 11b illustrates a dielectric salient feature 81 of therotating member passing into a portion of the field extending betweenthe electrode 79 and the rotating member 80 thereby condensing theequipotential lines and increasing the electric field gradient in thevicinity of the electrode. The increased gradient is equivalent to asituation where charge is brought near to the electrode causing currentto flow therein.

The dielectric member 81 allows charges to build up in the direction ofthe electrode 79. The molecules polarize within the dielectric inresponse to the field gradient and serve to counteract it. In adielectric with a dielectric constant higher than 1 (such as typicalplastics which are in the range 1.5 to 4) the equipotential lines areseparated within the material, and thus are necessarily closer togetherimmediately outside of the material.

The raised sensor voltage relative to a relevant ground permits thedescribed effect to be detected, and this system of measurement offersimproved signal to noise ratios. It has been discovered that voltageswell in excess of the typical voltages used in typical electronicscircuitry (i.e. 5V or 12V) causes effects in the movement of chargewhich are consistent with each revolution, whereas lower voltages allowthe sensor potential to fluctuate in response to contaminants in aproximate fluid flow, or any variation in potential of any relevantproximal ground due to poor electrical connections (such as due to ashaft or wheel being connected to earth through an oil film bearing).

The invention claimed is:
 1. A speed sensor for use in measuring thespeed of rotation of a rotationally salient rotating member, the speedsensor comprising an electrode and a sensor circuit; the sensor circuitcomprising: a constant voltage source configured to supply a constantvoltage without polarity reversal between the electrode and the rotatingmember to establish a potential difference between the electrode and therotating member and thereby establish an electric field which extends ina dielectric medium from the electrode to the rotating member; a currentdetector for detecting current flow between the constant voltage sourceand the electrode due to perturbation of the electric field by passageof at least one salient feature of the rotating member through theelectric field as the rotating member rotates, the current detectoroutputting a first signal modulated at a frequency corresponding to thefrequency of perturbation of the electric field; and an amplifiercircuit comprising a signal amplifier for amplifying the first signaland outputting a second signal modulated at a frequency corresponding tothe frequency of perturbation of the electric field such that the speedof rotation is determined based upon the frequency of the modulation ofthe second signal; wherein the sensor circuit further comprises acapacitor connected between the electrode and the signal amplifier, thecapacitor acting to isolate the signal amplifier from the constantvoltage applied to the electrode.
 2. A speed sensor according to claim1, wherein said constant voltage source supplies a voltage in the rangeof +/−30V to +/−1 kV.
 3. A speed sensor according to claim 1, wherein acable which extends from the electrode to the current detector iselectrically shielded by at least one conductive shield.
 4. A speedsensor according to claim 3, wherein the shield is held at a controlledelectric potential.
 5. A speed sensor according to claim 3, wherein saidamplifier circuit includes a shield amplifier the output of which drivesthe at least one conductive shield.
 6. A speed sensor according to claim5, wherein the at least one conductive shield includes at least twoconductive shields and the shield amplifier holds each conductive shieldat the same electric potential.
 7. A speed sensor according to claim 5,wherein the shield amplifier has substantially unity gain.
 8. A speedsensor according to claim 5, wherein the shield amplifier is coupled tothe signal amplifier.
 9. A speed sensor according to claim 8, whereinthe shield amplifier is connected in series with the signal amplifier,and comprising the feedback loop connected across the signal amplifierand said shield amplifier, the feedback loop including a secondcapacitor.
 10. A speed sensor according to claim 1, comprising afeedback loop connected across the signal amplifier, the feedback loopincluding a second capacitor.
 11. A speed sensor according to claim 10,wherein the second capacitor is part of capacitive divider which isadapted to reduce the effective capacitance of the second capacitor, thecapacitive divider comprising first and second resistors connected inseries with a connection being provided from the second capacitor to anode between the first and second resistors.
 12. A speed sensoraccording to claim 11, wherein a further resistor is connected in seriesbetween the second capacitor and the node between the first and secondresistors.
 13. A speed sensor according to claim 1, wherein the firstsignal is a modulated voltage, and wherein the current detectorcomprises a second capacitor and a resistor connected in series acrossthe voltage source, wherein the electrode is connected to a node betweenthe voltage source and a first terminal of the capacitor, and the firstoutput signal is taken from a node between a second terminal of thesecond capacitor and the resistor.
 14. A speed sensor according to claim1, wherein the electrode has as surface from which the electric fieldextends, at least a portion of that surface which in use will be closestto the rotating member being covered by body of protective material. 15.A speed sensor according to claim 14, wherein said protective materialis non-conductive.
 16. A speed sensor according to claim 1, wherein theelectrode comprises an electrode body and an electrode surface fromwhich the electric field extends, at least a portion of that surfacewhich in use will be closest to the rotating member being defined by aconductive material different from the body of the electrode.
 17. Thespeed sensor according to claim 1, wherein the electrode is separatedfrom the rotating member by a distance which is less than one third ofthe diameter of the rotating member.
 18. A rotating machine comprising:a rotationally salient rotating member supported for rotation about anaxis and a speed sensor for sensing the rotational speed of the rotatingmember, the speed sensor comprising: an electrode fixed in positionrelative to said axis; a constant voltage source connected to supply aconstant voltage without polarity reversal between the electrode and therotating member to establish a potential difference between theelectrode and the rotating member and thereby establish an electricfield which extends in a dielectric medium from the electrode to aportion of the rotating machine which is held at a substantiallyconstant potential, wherein the electrode is positioned so that at leastone salient feature of the rotating member passes through the electricfield as the rotating member rotates about said axis, thereby perturbingthe electric field on each revolution of the rotating member; a currentdetector for detecting current flow between the voltage source and theelectrode in response to said perturbation of the electric field; and anamplifier circuit comprising a signal amplifier for amplifying the firstsignal and outputting a second signal modulated at a frequencycorresponding to the frequency of perturbation of the electric fieldsuch that the rotational speed of the rotating member is determinedbased upon the frequency of the modulation of the second signal; whereina capacitor is connected between the electrode and the signal amplifier,the capacitor acting to isolate the signal amplifier from the constantvoltage applied to the electrode.
 19. A rotating machine according toclaim 18, wherein the portion of the rotating machine which is held atthe substantially constant potential is said rotating member.
 20. Arotating machine according to claim 18, wherein the rotating member isnon-conductive and said constant potential portion of the rotatingmachine is a conductive body supported by the rotating machine in afixed spatial relationship to the electrode so that said at least onesalient feature of the rotating member passes between the electrode andthe constant potential portion on each revolution of the rotating memberabout said axis.
 21. A rotating machine according to claim 20, whereinsaid constant potential portion of the rotating machine is a conductiveportion of a housing of the rotating machine.
 22. A rotating machineaccording to claim 20, wherein the rotating member is mounted on a shaftat least a portion of which is conductive and comprises said constantpotential portion of the rotating machine.
 23. The rotating machineaccording to claim 18, wherein said constant voltage source supplies avoltage in the range of +/−30V to +/−1 kV.
 24. A method of measuring thespeed of a rotating machine comprising a rotationally salient rotatingmember supported for rotation about an axis, the method comprising:supporting an electrode in a position relative to said axis; supplying aconstant voltage without polarity reversal between the electrode and therotating machine to establish a potential difference between theelectrode and the rotating machine and thereby establish an electricfield which extends from the electrode to a substantially constantpotential portion of the rotating machine; wherein the electrode ispositioned so that at least one salient feature of the rotating memberpasses through the electric field as the rotating member rotates aboutsaid axis, thereby perturbing the electric field on each revolution ofthe rotating member; detecting current flow between the voltage sourceand the electrode in response to said perturbation of the electric fieldto derive a first signal modulated at the frequency of saidperturbation; using a signal amplifier to amplify the first signal andoutputting a second signal modulated at a frequency corresponding to thefrequency of perturbation of the electric field; and determining therotational speed from the modulation of said signal; wherein the signalamplifier is isolated from the voltage applied to the electrode by acapacitor connected between the electrode and the signal amplifier. 25.The method according to claim 24, wherein said constant voltage sourcesupplies a voltage in the range of +/−30V to +/−1 kV.